To provide a liquid fuel concentration sensor which inhibits external leakage of liquid fuel, outside infiltration of impurities into liquid fuel and elution of impurities into liquid fuel and improve the measurement accuracy of the liquid fuel.

Th liquid fuel concentration sensor is equipped with a substrate 34 and a surface acoustic wave device 31, which includes surface acoustic wave electrodes 37a/37b prepared on surface side of the substrate 34; internal electrodes 38a, 38b prepared at the rear side of the substrate 34; and electric wires 40a, 40b passing through a through-hole 39 that connects each of surface acoustic wave electrodes 37a, 37b to internal electrodes 38a, 38b. The surface acoustic wave device 31 is arranged along the inner faces of plastic mounting members 13, 19 and 20 that internally house the liquid fuel and are such that surface acoustic wave propagation regions 35a, 35b approach the liquid fuel, while the internal electrodes 38a, 38b are interposed between the substrate 34 and the plastic mounting members 13, 19 and 20.